The middle ear is vital to auditory function. Middle-ear disease is the most common cause of hearing loss, and a wide range of treatments have been developed. Ear surgeons routinely repair or reconstruct middle-ear parts or the whole. However, the success of these reconstructions is limited, especially after more severe middle-ear disease. Knowledge of structure-function relations is crucial to improvements in reconstructive techniques. We have developed physiologically-based models that address issues relating to: normal ears, e.g. the function of the pars flaccida of the tympanic membrane (TM); pathological ears, e.g. perforations of TM; and reconstructions, e.g. stapedectomy and type IV tympanoplasty. We propose next to focus on four important structure-function issues: (1) A newly described clinical entity, superior-semicircular-canal dehiscence syndrome, is associated with altered sensitivity for air-conducted and bone-conducted sound, and vertigo precipitated by loud sounds or static pressure in the ear canal. We seek the mechanisms responsible for the hearing changes in an animal model where we produce canal dehiscences of varied size and location. Results could define a hearing test that indicates size and location of the dehiscence. (2) Loss of hearing sensitivity caused by static pressure in the middle ear is common. The loss in hearing is assumed to result from the nonlinear elastic properties of the TM and middle-ear ligaments, but the relative importance of these structures is unknown. Features are: (a) Positive and negative pressures have qualitatively different effects, and (b) Effects on admittance differ in magnitude from those on transmission. This project will determine reasons for these features and the structures that produce them. (3) Middle-ear response at high frequencies is often assumed to be limited by inertial forces, but measurements do not support this assumption. We propose measurements in gerbil and in several species of the cat family, where species of different size (small cats to tiger) provide natural variations in the size of middle-ear structures. Results should define important mechanisms involved in high-frequency middle-ear function. (4) Recent work has demonstrated a novel smooth-muscle element at the annulus of the TM. We will quantify this muscle's effect on the acoustic properties of the middle ear and assess its potential as a site for CNS control of middle-ear transmission.